This invention relates to a process for recovering starch and protein of high quality from a subterranean stem.
Subterranean stem starch is produced by crushing a subterranean stem of potato including sweet potato, or the like, separating starch granules in its cells therefrom, and then purifying the same by washing with water. In general, about 20% by weight or less of the raw subterranean stem is recovered as starch, and the residue, i.e., about 80% or more thereof is discharged as juice and fiberous residue (pulp). Recently, there are not a few factories having a processing capacity of 1,000 tons per day, and the amount of wastes produced in these large-scale factories is huge. In particular, the juice is a concentrated waste solution having an organic substance concentration of 2 to 5% including protein, and in the case of potato, protein is contained in the juice in an amount of about 2% of the organic substances.
Attempts to recover the protein in the juice have also been made. According to our research, the most suitable method is heat treatment of the waste assumed to be caused by actions of a group of enzymes which function as catalysts for polymerization reactions of low-molecular-weight aromatic compounds, namely, tyrosinase, catecholase, phenolase and polyphenol oxidase, but the reaction mechanism at the time of such crushing of the subterranean stem is not yet apparent. For these reasons, it becomes necessary for recovery of starch having a high whiteness to carefully remove the darkened juice and colored precipitate (low grade starch precipitate) other than starch by washing. Needless to say, the juice freed from pulp is also darkened and when it is heated to agglomerate protein, it becomes a dark grayish black floc, so that no white protein can be obtained. The color cannot be removed even by repeated washing and reprecipitation of these precipitates. Therefore, only low-grade protein having limited application can be obtained even by achieving such recovery.
Japanese Patent Appln Kokai (Laid-Open) No. 63200/76 (a process for producing water-soluble starch from potato juice) discloses that uncolored protein can be obtained by subjecting potato juice to ultrafiltration and then drying the concentrate thus obtained at a temperature at which the protein is not denatured. However, according to our study, a crushed potato slurry was colored in several seconds to tens of seconds and starch and protein separated from the colored slurry had already been colored, so that no uncolored protein, in particular, could be obtained. Since the amount of potato slurry is very large in a practical potato starch-manufacturing plant, it is very difficult to separate and recover protein by the heat treatment described above before the protein is colored.
The above patent appln Kokai discloses that potato juice is heated (50.degree. to 60.degree. C.) in a short time by steam injection to inactivate the enzymes or precipitate the colored substance. However, since a potato slurry is colored in a very short time as described above, it is difficult in practice to heat potato juice to 50.degree. to 60.degree. C. before coloring of a large amount of the slurry.